as long as the aforementioned conditions on \(\Phi \)  and \(b\)  hold. This estimate was first noticed in [1]} and, later, proved by generalised Radon transform methods in [2]}, [3]}. Such conditions can be relaxed a bit, but, such regularisation will not hold for \(b\in L^{1}(\mathbb {S}^{d-1})\) .
